Certain electric vehicles for transportation purposes are conventional. To achieve an acceptable range of transportation for these vehicles they have to be configured to carry and to provide the needed electrical energy to the drive train. Conventional cars have a gasoline tank in which the transportation energy is carried, and that energy is stored in the form of gasoline. The car can not drive any further if the tank is completely empty. In this case, no gas is left in the tank.
A battery system of an electric vehicle principally behaves differently than a conventional gas tank for storing energy. In an electric vehicle with multiple battery cells, the energy needed for driving is stored electrochemically in these cells. In use, the cells are usually not completely discharged or charged because this would harm the expected lifetime of the cells or even destroy the cells completely.
More importantly, continuing to charge a cell that is completely full or to discharge a cell past its recommended minimum voltage may damage the cell. In order to optimize the use and lifetime of the battery system and the useable amount of energy in the cells, a voltage balancing is usually performed on the batteries. In order to achieve an appropriate current for the traction system, the battery cells can be grouped in parallel. For balancing the battery system, the parallel group of cells, which—by definition for the present context—could include one or more cells, is provided with a discharge resistor or any other electric device capable of changing the energy content of the cells by discharging or even charging the parallel group of batteries.
The parallel group of cells are then connected in series to build a module, for example consisting of 7 to 16 parallel groups of cells. The modules are usually adapted to be replaceable in the field. It would be too difficult to replace single cells in the field. The module therefore is arranged in size and energy to serve as a field replaceable unit. The module might have one or more module controllers which are connected to a superior battery management system. To achieve a voltage level suitable and intended by the electric configuration for the electric drive train of the vehicle an appropriate number of modules is connected in series thereby adding up the voltage of the parallel groups of cells. The total stored energy in the battery cells of the battery system that can be discharged and used for the power train then determines the achievable driving range for a specific vehicle arrangement.
The cells used in the modules are usually all of the same kind but still their electrical characteristics are not exactly identical. Some reasons for the difference are: The cells might originate from different production batches, might differ in production tolerances or have experienced different environmental impacts leading to different aging processes of the cells. Usually charged batteries continuously lose a small amount of their stored energy over time by unwanted internal leakage. This energy leakage portion, for example, is also dependant on the temperature of the cells and of the power consumption of the electronics connected to the cells. If all batteries in the battery system are charged to the same level, then, after a certain time, the amount of energy left in each battery is different because of their individual internal leakage. If the batteries are discharged, for example, while driving the electric vehicle, the battery system can only be discharged to the lowest allowable state of charge of the parallel group of batteries with the lowest state of charge. The state of charge is defined as a measure of the present charge of the cell compared to its maximum charge. In this situation, it is likely that there are groups of parallel batteries which are not empty yet but which can not be discharged further because one parallel group of batteries in a module has reached its lowest allowable state of charge. In this situation, there is still energy in the battery system left that can not be used because of an imbalance of state of charge of the parallel groups of batteries. This leads to an unwanted reduced possible mileage of the electric vehicle. Similar arguments apply to charging the battery system. The parallel group of cells with the highest state of charge will terminate the charging step, because this group can not be charged anymore without harming the corresponding parallel group of cells. Therefore an unbalanced battery has at the same time unused capacity in parallel groups of cells with lower states of charge.
Certain conventional methods constantly or repeatedly track the voltage level of the batteries and constantly evaluate when balancing circuits should be turned on. Examples of this are described, for example, in U.S. Pat. No. 6,275,004, U.S. Pat. No. 7,723,955, and U.S. Pat. No. 7,602,145.
Other methods of balancing are described in U.S. Pat. No. 5,998,967 and U.S. Pat. No. 6,844,703 and involve balancing during charging by controlling the charging current to individual batteries such that all the batteries reach the same voltage at the end of the charge.
U.S. Patent Application Publication No. 2009/0302685 describes a possibility of using no uniform batteries as part of one central energy storage system. All batteries are connected to a central node via energy transfer circuit.
Also conventional is the integrated circuit LTC6802-2 from Linear Technology for battery monitoring as well as battery balancing purposes. The integrated circuit chip is capable of measuring the voltage of 12 series connected battery cells. The LTC6802-2 includes circuitry to balance cell voltages, whereas it makes no decisions about turning on or off internal switches. For these decisions a host processor has to write values to a configuration register inside the LTC6802-2 to control the switches.